This invention relates to a high-strength silicon nitride sintered body suitable for heat resistant structural materials, machine tool materials especially cutting tool materials, wear resistant materials and corrosion resistant materials, and to a method for preparing the sintered body.
A silicon nitride compound has a strong covalent bond, decomposes and evaporates at a high temperature, is poor in a reactivity because of a small self diffusion coefficient of its constituent atoms, and has a greater ratio of a surface energy to a grain boundary energy, as compared with an ionic crystal and a metallic crystal, therefore the compound above is very difficult to sinter. When the silicon nitride is thus sintered in a usual pressureless sintering manner, any dense sintered body cannot be obtained. In order to produce the dense sintered body, a sintering auxiliary such as MgO, Y.sub.2 O.sub.3, Al.sub.2 O.sub.3 or AlN is generally used and a pressure sintering or a hot isostatic pressing method (HIP) is utilized under a reaction sintering or a liquid phase sintering.
In the Si.sub.3 N.sub.4 sintered body including sintering auxiliaries such as MgO, Y.sub.2 O.sub.3, Al.sub.2 O.sub.3 or AlN, a lower silicate is formed in grain boundary phases thereof. A liquid phase the lower silicate occur at a low temperature accelerating the sinterability of the Si.sub.3 N.sub.4. However, it remains in the grain boundary phases even after the sintering process, thereby disadvantageously lowering the strength of the sintered body at a high temperature. For the sake of overcoming such a drawback, a new method has been suggested by which the lower silicate remaining in the grain boundary phases of the Si.sub.3 N.sub.4 has been crystallized due to thermal treatment. Thereby, the strength of the sintered body increases. The lower silicate or second phases will be relatively uniformly dispersed in the grain boundary phases of the Si.sub.3 N.sub.4, when the Si.sub.3 N.sub.4 sintered body is tentatively sintered in a small size. In this case, thus, no serious problem has occurred. However, when a body having a complicated shape or great size is sintered on an industrial scale, the second phases mainly comprising the sintered auxiliary of an oxide will ununiformly be dispersed in the grain boundary phases of the Si.sub.3 N.sub.4 and a segregation will be brought about, because of a bad reactivity of the Si.sub.3 N.sub.4 with the sintering auxiliary, and because of the problem of a cooling rate at the time of using a large-scale sintering furnace. As understood from the foregoing, the bad reactivity of the Si.sub.3 N.sub.4 with the sintering auxiliary and the segregation of the second phases mainly comprising the sintered auxiliary will lead to an increased scattering of properties of the Si.sub.3 N.sub.4 sintered body and will cause a drop in the strength. Therefore, it is fair to say that an industrialization of such a suggested method is difficult from the viewpoint of technology.
This invention has thus been achieved with the intention of solving the above-mentioned drawbacks and problems, and its object is to provide a silicon nitride sintered body and a method for preparing it by which a reactivity of Si.sub.3 N.sub.4 with a sintering auxiliary is improved. Second phases uniformly disperse into the sintered body having an intricate shape or a great size, a bonding strength between the second phases and the Si.sub.3 N.sub.4 will increase and build up many properties such as strength, thermal resistance, wear resistance and toughness of the sintered body.